Efficiency is an important metric of high power amplifiers. In order to achieve maximum efficiency these amplifiers are often biased into strongly non-linear modes of operation. Modern high capacity modulation techniques exhibit large amplitude variation, which result in large distortion components on the output signal, leaking energy into neighboring frequency bands.
Predistortion techniques attempt to compensate for the power amplifier non-linearity by applying an inverse characteristic to the input signal. These techniques are very sensitive to the system gain, the variation of which can effectively un-calibrate the predistortion signal (the compensation applied to the signal reaching the non-linear component is either too large or too small) and degrade linearization performance.
Gain fluctuation with temperature, age, etc, is a well known phenomena associated with amplifiers and radio transceivers. AGC (Automatic Gain Control) loops have been widely used in radio systems for many years, ensuring optimum dynamic range, linearity and noise performance. The majority of these are circuit-based first- or second-order control loops, which are well understood in general control theory. Nonetheless the gain adjustment circuits in the AGC loops may themselves introduce undesired nonlinearity.
Therefore a need exists for an improved system and method for compensating for gain variations in the forward signal path of a transmitter employing a high power amplifier, which gain variations degrade linearization performance.